Put the stock, garlic, oil, rosemary, chilli and salt in a saucepan and bring to a simmer. Stirring constantly, slowly pour in the polenta. Keep stirring! The mixture will thicken quickly and bubble like a Rotorua mud pool. Watch out because it burns like nothing else if it lands on you.
Stir for 3-4 minutes, then stir through the olives and pour the mixture into a lined slice tin. Let it cool until it stops steaming then put in the fridge for an hour or so or until well set. You can also make this ahead of time and just leave it in the fridge until you're ready to grill it.
Turn the polenta out onto a chopping board and slice it into whatever shape your heart desires. I went with small triangles, which were easy to turn over in the pan and also looked pretty.
Heat a griddle pan or frypan over a high heat. If you're using a griddle pan you want it pretty scorching otherwise the polenta will stick. Rub a little olive oil on both sides of the polenta, then add to the pan. Let it sit without moving it until you're sure it's golden (or has grill marks, in the case of the griddle pan) then pray to the kitchen gods that it hasn't stuck, and flip it.
Once the second side is golden, remove from the pan and keep warm in a low oven until you're ready to serve.